I have two QNAPS on two different sites. Both have similar setups. On each site there is a management network and storage network. I wanted to test QNAP replication by backing up over an MPLS (storage network). The only problem is that the default gateway is needed for the management network. QNAP does not allow you to add another gateway on the secondary interface.
To achieve this we need to add routes to each QNAP in order to route the traffic over the MPLS network.
Log into QNAP A via SSH
route add -net 172.17.35.0 netmask 255.255.255.0 gw 192.168.35.254
Then log into QNAP B via SSH
route add -net 192.168.35.0 netmask 255.255.255.0 gw 172.17.35.254
Note: Ensure you log in as the 'admin' user via SSH.